The present invention relates to a DC-DC converter for converting a DC voltage into a DC output via a transformer and, in particular, to a self-oscillating DC-DC converter that can keep an output voltage constant by providing control as consisting of both frequency and pulse-width modulation in response to variations in an input voltage or a load.
FIG. 8 shows a conventional example of a self-commutated resonant converter. As shown in this FIGURE, a DC power supply 1, a capacitor 4, a primary coil 21 of a transformer 2, and a semiconductor switching device 91 are connected together in series: a parallel circuit of a semiconductor switching device 92 and a capacitor 5 is connected between the capacitor 4 and the primary coil 21 of the transformer in parallel; diodes 81 and 82 and a filter capacitor 3 are connected to secondary coils 22 and 23 of the transformer 2; and a DC output is connected to the gates of the semiconductor switching devices 91 and 92 via an output-voltage detection and control circuit 6, a frequency control circuit 14, and a high-voltage-resistant driver IC 15.
FIG. 9 shows an example of the operation of the converter illustrated in FIG. 8. References v92, v91, v4, and v21 denote voltage waveforms from the semiconductor switching device 92, the semiconductor switching device 91, the capacitor 4, and the primary coil 21 of the transformer, and references i92, i81, and i82 denote current waveforms from the semiconductor switching device 91, the semiconductor switching device 92, the diode 81, and the diode 82.
During a period {circle around (1)}, when the semiconductor switching device 91 is turned on, the resonant current i91 flows through the DC power supply 1xe2x86x92the capacitor 4xe2x86x92the primary coil 21 of the transformerxe2x86x92the semiconductor switching device 91 to charge the capacitor 4. At this time, the difference in voltage between the DC power supply and the capacitor 4 is applied to the primary coil 21 of the transformer to charge the filter capacitor 3 via the diode 81, while supplying power to a load.
During a period {circle around (2)}, when the semiconductor switching device 91 is turned off, the resonant current is commuted to the output capacities of the semiconductor switching devices 91 and 92 and the capacitor 5, thereby gradually raising or lowering the voltages at the semiconductor switching devices 91 and 92. During a period {circle around (3)}, once the voltage at the semiconductor switching device 91 reaches the DC power-supply voltage, the resonant current is commuted to a parasitic diode of the semiconductor switching device 92. At this time, when the semiconductor switching device 92 is turned on, the resonant current i92 flows through the capacitor 4xe2x86x92the semiconductor switching device 92xe2x86x92the primary coil 21 of the transformer to discharge the capacitor 4. Further, the difference in voltage between the DC power supply and the capacitor 4 is applied to the primary coil 21 of the transformer to charge the filter capacitor 3 via the diode 82, while supplying power to the load.
During a period {circle around (4)}, when the semiconductor switching device 92 is turned off, the resonant current is commuted to the output capacities of the capacitor 5 and the semiconductor switching devices 91 and 92, thereby gradually raising or lowering the voltages at the semiconductor switching devices 91 and 92. During the period, {circle around (1)}, once the voltage at the semiconductor switching device 92 reaches the DC power-supply voltage, the resonant current is commuted to a parasitic diode of the semiconductor switching device 91. At this time, when the semiconductor switching device 91 is turned on, such an operation is repeated to supply DC output power insulated from the DC power supply. The circuit illustrated in FIG. 8 operates as illustrated in FIG. 9, regardless of its load state (light or heavy load) or input voltage.
In the conventional example illustrated in FIG. 8, in response to variations in the load, the output-voltage detection and control circuit and the frequency control circuit are used to modulate the operating frequencies of the semiconductor switching devices, in order to keep the output voltage constant. This method is not based on the current commonly used pulse-width modulation method, and requires relatively expensive high-voltage-resistant driver ICs to drive the semiconductor switching device 92. Further, the frequency control circuit may be replaced by a pulse-width modulation circuit, and the high-voltage-resistant driver ICs may be replaced by pulse transformers, though the use of pulse transformers hinders size reduction.
It is thus an object of the present invention to eliminate the need for high-voltage-resistant driver ICs or pulse transformers in order to reduce costs.
To attain this object, the invention set forth in claim 1 provides xcex9 DC-DC converter for converting DC power from a DC power supply into an arbitrary DC output via a transformer, with the DC-DC converter being characterized in that:
the DC power source, a first capacitor, a primary coil of the transformer, a first semiconductor switching device, and a current-limiting resistor are connected together in series; a parallel circuit of a second semiconductor switching device and a second capacitor is connected between the first capacitor and the primary coil of the transformer in parallel; first and second transformer driving coils are each connected between a gate and a source of the first or second semiconductor switching device, respectively, via a resistor; an activation circuit and a transistor are connected between the gate and source of the first semiconductor switching device; the base of the transistor is connected to a connection between the first semiconductor switching device and the current-limiting resistor via a base resistor; a diode and a filter capacitor are connected to a secondary coil of the transformer; and a DC output is connected to the base of the transistor via an output-voltage detection and control circuit.
The invention set forth in claim 2 provides a DC-DC convertor for converting DC power from a DC power supply into an arbitrary DC output via a transformer, with the DC-DC converter being characterized in that:
the DC power source, a first capacitor, a primary coil of the transformer, a first semiconductor switching device, and a current-limiting resistor are connected together in series; a parallel circuit of a second semiconductor switching device and a second capacitor is connected between the first capacitor and the primary coil of the transformer in parallel; first and second transformer driving coil are each connected between a gate and a source of the first and second semiconductor switching device, respectively, via a resistor; an activation circuit and a transistor are connected between the gate and source of the first semiconductor switching device; the base of the transistor is connected to a connection between the first semiconductor switching device and the current-limiting resistor via a base resistor; a first diode is connected to one terminal of a first secondary coil of the transformer so as to supply power when a positive voltage is applied to the primary coil of the transformer; a second diode is connected to one terminal of a second secondary coil of the transformer so as to supply power when a negative voltage is applied to the primary coil of the transformer; cathodes of the first and second diodes are connected to one terminal of a filter capacitor; the other terminals of the first and second coils of the transformer are both connected to the other terminal of the filter capacitor; and a DC output is connected to the base of the transistor via an output-voltage detection and control circuit.
In the invention set forth in claim 2, magnetic coupling between the primary coil of the transformer and the first secondary coil of the transformer is closer than that between the primary coil of the transformer and the second secondary coil of the transformer (invention set forth in claim 3), or magnetic coupling between the primary coil of the transformer and the second secondary coil of the transformer is closer than that between the primary coil of the transformer and the first secondary coil of the transformer (invention set forth in claim 4).